1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems include a network of base stations, base station routers, and/or other wireless access points that are used to provide wireless connectivity to access terminals in geographic areas (or cells) associated with the network. Information may be communicated between the network and the access terminals over an air interface using wireless communication links that typically include multiple channels. The channels include forward link (or downlink) channels that carry signals from the base stations to the access terminals and reverse link (or uplink) channels that carry signals from the access terminals to the base station. The channels may be defined using time slots, frequencies, scrambling codes or sequences, or any combination thereof. For example, the channels in a Code Division Multiple Access (CDMA) system are defined by modulating signals transmitted on the channels using orthogonal codes or sequences. For another example, the channels in an Orthogonal Frequency Division Multiplexing (OFDM) system are defined using a set of orthogonal frequencies known as tones or subcarriers.
Next (4th) generation wireless systems such as 802.16e WiMAX, UMTS Long Term Evolution (LTE) and cdma2000 EV-DO Revision C Ultra Mobile Broadband (UMB) are based on Orthogonal Frequency Division Multiple Access. In OFDMA, the transmitted signal consists of narrowband tones that are nearly orthogonal to each other in the frequency domain. A group of tones transmitted over the duration of one time slot (or frame) constitutes the smallest scheduling resource unit, also known as a tile, a resource block (RB), or a base node (BN). Different tones belonging to a tile may be scattered across the entire carrier frequency band used by the OFDMA system so that each tile transmission experiences diversified channels and interference on each sub-carrier. Alternatively, a tile can be formed of a contiguous set of tones so that the channel and interference experienced by the tile are more localized. Hybrid Automatic Repeat reQuest (HARQ) is employed to increase the capacity of the OFDMA system. To this end, the encoder packet transmission includes multiple HARQ interlaces repeating every certain number of frames and having a fixed maximum allowed number of sub-packet retransmissions.
OFDMA is a fully scheduled medium access control scheme on both the uplink and downlink channels. For example, in systems such as UMB that employ synchronous HARQ, a scheduler allocates tile-interlace resources for the duration of each encoder packet transmission. Many existing scheduler algorithms for OFDMA systems focus on downlink scheduler design and rarely address the design of the uplink scheduler. The few conventional uplink scheduler designs that have been discussed in the literature do not consider the constraints imposed by interference control. In particular, designs for conventional uplink schedulers fail to consider the constraint on the maximum transmit power spectral density of a access terminal imposed by interference power level control. The conventional uplink scheduler designs also fail to describe the process of packet format selection. Instead, existing uplink scheduler algorithms often assume that the access terminals all request a specific pre-determined data rate. Conventional uplink schedulers also do not consider packet format selection or the coding and modulation selection based on the RF channel condition fed back from the access terminals. Therefore, a degree of freedom for uplink scheduler design is missing from conventional uplink scheduler designs.